This invention relates to a process for making a lithographic printing plate, more specifically to a process for making a lithographic printing plate which can be handled under a room light, can image with a laser under heat-mode, can provide an image with high resolution and generates no waste liquid by the plate-making method of a lithographic printing material.
A lithographic printing plate comprises an oleophilic image area which accepts oily ink, and an oil-repellent non-image area which does not accept ink. The non-image area generally comprises a hydrophilic portion which accepts water. In a usual lithographic printing plate, printing is carried out by supplying both of water and an ink to a plate surface whereby the image area selectively accepts the ink and the non-image area selectively accepts water, and transferring the ink on the image area to a material to be printed such as paper, or the like.
A lithographic printing plate has now been prepared by forming an oleophilic ink receptive layer on a substrate such as an aluminum plate, a zinc plate, paper or the like, the surface of which are subjected to hydrophilic treatment. Among these, it is general to use those in which a photosensitive material such as a diazo compound or a photopolymer, etc. is provided on an aluminum base the surface of which is subjected to hydrophilic treatment which is so-called a presensitized (PS) plate, or those in which an image is formed by utilizing a silver complex diffusion transfer process (the DTR method) using a silver halide on paper or a plastic support as a photosensitive material.
A method for forming an ink receptive layer (hereinafter referred to as "an image layer") by a diazo compound or a photopolymer comprises firstly coating a photosensitive material such as a diazo compound, a photopolymer or the like on a substrate such as a metal plate, paper, a laminated plate, an insulating substrate, or the like. Then, light is exposed to the photosensitive material to cause chemical change whereby dissolution properties to a developing solution are changed. Here, the photosensitive material can be divided into two types depending on the kind of the chemical change. That is, one is a negative type in which the portion to which light is exposed is polymerized and cured to become insoluble to a developing solution, and the others is a positive type in which a functional group at the portion to which light is exposed is changed to have solubility in a developing solution. In either of the case, the photosensitive material insoluble in a developing solution, which remains on the substrate after processing with a developing solution, becomes an image layer.
On the other hand, a lithographic printing plate using the DTR method, particularly a lithographic printing plate having a physical development nuclei layer on a silver halide emulsion layer is described in, for example, U.S. Pat. Nos. 3,728,114, No. 4,134,769, No. 4,160,670, No. 4,336,321, No. 4,501,811, No. 4,510,228 and No. 4,621,041. Exposed silver halide microcrystals cause chemical development by the DTR developer and change to black silver to form a hydrophilic non-image area. On the other hand, unexposed silver halide microcrystals become silver complex by a complexing agent in a developing solution to diffuse to a physical development nuclei layer at the surface thereof and cause physical development in the presence of a nucleus whereby an image area mainly comprising ink receptive physically developed silver is formed. Also, a lithographic printing plate in which a physical development nuclei layer and a silver halide emulsion layer are coated in this order on a grained and anodized aluminum support is disclosed in, for example, Japanese Provisional Patent Publications No. 260491/1988, No. 116151/1991, No. 282295/1992 or the like. The above-mentioned lithographic printing plate is imagewisely exposed, followed by DTR development, the silver halide emulsion layer is washed with warm water to form an image area mainly comprising the physically developed silver on the anodized aluminum base.
As a plate-making step of the process for making these printing plates, in the past, it was a main stream to prepare an intermediate film or a block copy from letters, images or photographic copies, gathering these to a plate to prepare a finishing film and then to use close contact exposure system with an ultraviolet ray or a white light. Also, a method of laminating block copies to prepare a complete block copy and photographing it with a camera for plate making has been used. However, accompanying with the progress of computer technology, it has been employed a laser direct imaging system in which digital signal from computer information is transferred to an exposure device (computer to plate) and a photosensitive material is directly exposed by using a laser. The laser direct imaging system has advantages of low cost, time-saving, high productivity in many kinds of products with a small size or the like since films to be used in the course of the processing can be omitted.
In order to be applied with the laser direct imaging system, a photosensitive material having a high sensitivity is preferably used. In a diazo compound or a photopolymer, their sensitivities are low as several to several hundreds mJ/cm.sup.2 since a photochemical reaction occurs. Thus, to use the material for exposure, a laser output apparatus is required to be high power output whereby there are problems that the apparatus becomes large or a cost becomes expensive.
On the other hand, when an image is formed by the DTR method using a silver halide, its sensitivity is several .mu.J/cm.sup.2 so that exposure can be carried out sufficiently with a simple and easy semiconductor laser or the like. However, there is a defect that efficiency and plate-making operations become markedly poor since storage, coating to a substrate or the like before subjecting to exposure must be carried out under a safety light. Further, in the case of the diazo compound or the photopolymer, the reaction proceeds by a room light or sunlight and their reactivities change at a high temperature. Moreover, when an oxygen is present, it becomes an inhibitor of the reaction. Thus, it is necessary to treat under a dark room or store at a low oxygen condition until exposure and development are carried out.
Furthermore, in the above image formation methods, it is general to carry out processing with a liquid using a developing solution so that there is a defect that treatment of the waste solution becomes environmental issue. From 1995, a waste liquid is prohibited to throw into the ocean and processing with a dried state is earnestly desired today.
As a material which satisfies the above demand, it has been proposed a printing plate of a system in which image formation is carried out by providing an oleophilic metal thin layer on a support having a hydrophilic surface and imagewisely removing the oleophilic metal thin layer with irradiation of a high output heat-mode laser beam. For example, in Japanese Provisional Patent Publication No. 180976/1998, it is disclosed a process for making a plate by effecting a heat-mode laser exposure of a printing plate precursor in which a silver thin layer is formed on a support having a hydrophilic layer by DTR development without effecting development.
For making such a printing plate precursor, the silver complex diffusion transfer process is effective as a method for forming a silver thin film on a support. As the first system, there is a system in which a material wherein a physical development nuclei layer is provided on a support and a material wherein a silver halide emulsion layer is coated on a support as a donor for a silver complex salt are passed through a physical developer, piled up and subjecting to diffusion transfer development to precipitate silver on physical development nuclei to form a silver thin layer. Examples of the material using the system of forming a silver thin film as mentioned above may include Copy rapid (trade name, available from Agfa Gevaert) or the like.
As the second system, a material, in which a silver halide emulsion layer is provided as a donor for the silver complex salt on a support to which a physical development nuclei layer is provided, is subjected to physical development processing, and then, the silver halide emulsion layer is washed-off to form a silver thin film on the physical development nuclei. Examples of employing such a system to form a silver thin film may include Silver Digiplate SDP-.alpha.R (trade name, available from Mitsubishi Paper Mills Ltd.), Silverlith SDB (trade name, available from E.I. Du Pont) or the like.
However, according to the above two-types of the systems, there are problems that preparation of the silver halide emulsion layer is troublesome, preparation of the silver thin film requires dark room and the like.
As the third system, a support to which a physical development nuclei layer is provided is immersed in a solution containing a silver complex salt dissolved by a silver halide solvent, and a reducing agent, to form a silver thin layer on the physical development nuclei. This system has been also known as the electroless plating system, and disclosed in, for example, Japanese Patent Publications No. 23745/1967 and No. 12862/1968, and Japanese Provisional Patent Publication No. 287542/1993.
However, according to these methods disclosed in the prior art references, there are drawbacks that desired printability cannot be obtained, a silver film is required to be formed within a shorter time and the like.